Technical Field
[0001] The present invention relates to a projection lens.
Background Art
[0002] In recent years, imaging performance required for a projection lens has been increased
as the number of pixels of a projected image increases, and the number of constituent
lenses tends to increase. Regarding this tendency, in a case of a conventional projection
lens including 15 lenses each having four antireflective films formed on a surface
of a lens substrate having, for example, a refractive index of 1.52, a light reflection
loss of about 5% in the entire projection lens is generated on average in a visible
light wavelength range. Furthermore, in a case of a projection lens including 30 lenses,
there is a concern that a light reflection loss of about 10% is generated, and brightness
of an image projected on a projection plane is largely reduced. Therefore, in order
to suppress a decrease in the transmittance of the whole system of a projection lens
in response to an increase in the number of constituent lenses, an antireflective
film with a lower reflectance and a smaller loss of light is required for a lens substrate.
An example of conventional technology related to solving this problem is disclosed
in
JP 2002-267803 A.
[0003] In an antireflective film described in
JP 2002-267803 A, in order from a substrate side, a first layer is formed of a material having a refractive
index lower than that of the substrate, second, fourth, sixth, and eighth layers are
formed of a high refractive index material, third, fifth, seventh, and ninth layers
are formed of a low refractive index material, and the optical film thickness of each
of the layers is individually set to a predetermined value related to a design wavelength.
This prevents reflection in a wide wavelength band from an ultraviolet region to an
infrared region.
Summary of Invention
Technical Problem
[0005] However, according to the conventional technology described in
JP 2002-267803 A, even in an embodiment having the lowest maximum reflectance in a visible light wavelength
range (for example, 420 nm to 690 nm), the maximum reflectance is about 0.5%, which
is relatively high. As a result, it is a problem that a projection lens is insufficient
for application to a recent increase in the number of pixels of an image.
[0006] The present invention has been achieved in view of the above points. An object of
the present invention is to provide a projection lens capable of effectively suppressing
a decrease in the transmittance of the whole system and capable of coping with an
increase in the number of constituent lenses.
Solution to Problem
[0007] In order to solve the above problems, the present invention provides a projection
lens for projecting an image onto a projection plane, including: a lens substrate;
and an antireflective film constituted by at least eight layers, formed on a surface
of the lens substrate, characterized in that, in the antireflective film, in order
from an air side, a first layer is formed of MgF
2, each of a second layer, a fourth layer, a sixth layer, and an eighth layer has a
refractive index of 2.0 to 2.3, each of a third layer, a fifth layer, and a seventh
layer is formed of SiO
2, and quarter wave optical thicknesses Q
1 to Q
8 for the first layer to the eighth layer with respect to a refractive index n
s of the lens substrate at a design main wavelength of λ
0 = 550 nm satisfy the following formulas (1) to (8).
[0008] In addition, the projection lens having the above configuration is characterized
in that each of the second layer, the fourth layer, the sixth layer, and the eighth
layer is formed of any one of Ta
2O
5, LaTiO
3, a mixture of Ti
2O
3 and ZrO
2, and a mixture of ZrTiO
4 and ZrO
2.
[0009] In addition, the projection lens having the above configuration is characterized
in that the antireflective film has a maximum reflectance of 0.2% or less in a wavelength
range of 430 nm to 670 nm.
Advantageous Effects of Invention
[0011] According to the present invention, it is possible to form an antireflective film
with a low reflectance and a small loss of light corresponding to lens substrates
having various refractive indices. That is, it is possible to form an antireflective
film using a high refractive index material which has been relatively difficult to
use conventionally, and it is possible to expand the degree of freedom of the configuration
of a projection lens. As a result, it is possible to effectively suppress a decrease
in the transmittance of the whole system of a projection lens, and it is possible
to flexibly cope with an increase in the number of constituent lenses.
Brief Description of Drawings
[0012]
Fig. 1 is an optical configuration diagram of a projection lens according to an embodiment
of the present invention.
Fig. 2 is a layer configuration diagram of an antireflective film of a single lens
of the projection lens according to the embodiment of the present invention.
Fig. 3 is a graph illustrating spectral reflectance characteristics of an antireflective
film of a lens substrate in Comparative Example with respect to the embodiment of
the present invention.
Fig. 4 is a graph illustrating spectral reflectance characteristics of an antireflective
film of a lens substrate in Example 1 of the projection lens according to the embodiment
of the present invention.
Fig. 5 is a graph illustrating spectral reflectance characteristics of an antireflective
film of a lens substrate in Example 2 of the projection lens according to the embodiment
of the present invention.
Fig. 6 is a graph illustrating spectral reflectance characteristics of an antireflective
film of a lens substrate in Example 3 of the projection lens according to the embodiment
of the present invention.
Fig. 7 is a graph illustrating spectral reflectance characteristics of an antireflective
film of a lens substrate in Example 4 of the projection lens according to the embodiment
of the present invention.
Fig. 8 is a graph illustrating spectral reflectance characteristics of an antireflective
film of a lens substrate in Example 5 of the projection lens according to the embodiment
of the present invention.
Fig. 9 is a graph illustrating spectral reflectance characteristics of an antireflective
film of a lens substrate in Example 6 of the projection lens according to the embodiment
of the present invention.
Description of Embodiments
[0013] Hereinafter, an example of an embodiment of the present invention will be described
with reference to the drawings.
[0014] First, the configuration of a projection lens according to the embodiment of the
present invention will be described with reference to Fig. 1. Fig. 1 is an optical
configuration diagram of a projection lens, illustrating the lens cross-sectional
shape, the lens arrangement, and the like of a projection lens LN with optical cross
sections at each of a wide-angle end (W) and a telephoto end (T). The right side of
Fig. 1 is a reduction side of the projection lens LN, and the left side of Fig. 1
is an enlargement side of the projection lens LN. Note that a prism PR (for example,
a total internal reflection (TIR) prism or a color separation/synthesis prism) and
a cover glass CG of an image display element are illustrated on the reduction side
of the projection lens LN.
[0015] The projection lens LN is constituted by, for example, 30 lens components as a whole
as illustrated in Fig. 1, and includes a first optical system LN1 and a second optical
system LN2 in order from the enlargement side with an intermediate image plane IM1
interposed therebetween. The second optical system LN2 forms an intermediate image
of an image displayed on an image display surface IM2 of an image display element
on the intermediate image plane IM1. The first optical system LN1 enlarges and projects
the intermediate image. Note that an aperture stop ST is located near the center of
the second optical system LN2 in an optical axis direction. A relay lens is used in
order to obtain both a wide field angle and excellent projection performance. Therefore,
the number of constituent lenses is large.
[0016] Next, the detailed configuration of a single lens used for the projection lens LN
will be described with reference to Fig. 2. Fig. 2 is a layer configuration diagram
of an antireflective film of a single lens.
[0017] A lens 1 used for the projection lens LN includes a lens substrate 10 and an antireflective
film 20 illustrated in Fig. 2. The lens substrate 10 is constituted by a transparent
substrate formed of, for example, glass (glass material). On a surface of the lens
substrate 10, the antireflective film 20 constituted by eight layers is formed.
[0018] The antireflective film 20 includes, in order from an air side, a first layer 21,
a second layer 22, a third layer 23, a fourth layer 24, a fifth layer 25, a sixth
layer 26, a seventh layer 27, and an eighth layer 28. The first layer 21 is formed
of MgF
2. Each of the second layer 22, the fourth layer 24, the sixth layer 26, and the eighth
layer 28 is formed of a so-called high refractive index material having a refractive
index of 2.0 to 2.3. Each of the third layer 23, the fifth layer 25, and the seventh
layer 27 is formed of SiO
2.
[0020] Each of the layers of the antireflective film 20 is formed by a vacuum deposition
method under heating, for example, at 300°C. Particularly, the second layer 22 to
the eighth layer 28 are formed by a vacuum deposition method using ion assist. Ion-assisted
vapor deposition is desirably used in order to reduce a change in film density of
the antireflective film 20 and the roughness of a film surface due to variation of
the degree of vacuum and the like in the vacuum deposition method. This makes it possible
to suppress occurrence of color unevenness and deterioration of characteristic reproducibility
caused by a change in film density, that is, a change in refractive index of a film.
When ion-assisted vapor deposition is used for forming the antireflective film 20,
it is possible to use a high refractive index material which has been relatively difficult
to use conventionally for the layers constituting the antireflective film 20.
[0021] According to the above configuration, the antireflective film 20 has a maximum reflectance
of 0.2% or less in a wavelength range of 430 nm to 670 nm.
[0022] Note that each of the second layer 22, the fourth layer 24, the sixth layer 26, and
the eighth layer 28 in the antireflective film 20 is preferably formed of any one
of Ta
2O
5, LaTiO
3, a mixture of Ti
2O
3 and ZrO
2, and a mixture of ZrTiO
4 and ZrO
2.
Examples
[0024] Subsequently, regarding the present embodiment, evaluation of light reflectance of
a lens substrate and an antireflective film in each of Examples and a lens substrate
and an antireflective film in Comparative Example will be described with reference
to Figs. 3 to 9. Fig. 3 is a graph illustrating spectral reflectance characteristics
of an antireflective film of a lens substrate in Comparative Example. Figs. 4 to 9
are graphs illustrating spectral reflectance characteristics of antireflective films
of lens substrates in Examples 1 to 6. Note that the vertical axis indicates reflectance
and the horizontal axis indicates wavelength of light in Figs. 3 to 9.
[0025] Conditions of a glass substrate and an antireflective film in Comparative Example
are illustrated in Table 1. In Comparative Example, a general antireflective film
constituted by four layers was formed on a surface of a glass lens substrate having
a refractive index n
s = 1.52 at a design main wavelength of λ
0 = 550 nm. Each layer of the antireflective film was formed by a vacuum deposition
method under heating at 300°C.
[Table 1]
Comparative Example |
Design main wavelength λ0 = 550 [nm] |
Material for layer |
QWOT |
First layer |
MgF2 |
0.93 |
Second layer |
LaTiO3 |
1.86 |
Third layer |
AL2O3 |
1.09 |
Fourth layer |
MgF2 |
0.41 |
Lens substrate Refractive index ns: 1.52 |
[0026] According to Fig. 3 illustrating the spectral reflectance characteristics of the
lens substrate and the antireflective film in Comparative Example, a maximum reflectance
in a visible light wavelength range of 430 nm to 670 nm was 0.26%. It is found that
Comparative Example has a relatively high maximum reflectance.
[0027] Conditions of the glass substrate 10 and the antireflective film 20 in Example 1
are illustrated in Table 2. In Example 1, the antireflective film 20 constituted by
eight layers was formed on a surface of the glass lens substrate 10 having a refractive
index n
s = 1.52 at a design main wavelength of λ
0 = 550 nm. The quarter wave optical thicknesses (QWOT) Q
1 to Q
8 of the first layer 21 to the eighth layer 28 with respect to a refractive index n
s = 1.52 of the lens substrate 10 at a design main wavelength of λ
0 = 550 nm satisfy the following formulas (1) to (8). Each layer of the antireflective
film 20 was formed by a vacuum deposition method under heating at 300°C. Particularly,
the second layer 22 to the eighth layer 28 were formed by a vacuum deposition method
using ion assist.
[Table 2]
Example 1 |
Design main wavelength λ0 = 550 [nm] |
Material for layer |
QWOT |
First layer |
MgF2 |
Q1 |
0.94 |
Second layer |
Ta2O5 |
Q2 |
1.89 |
Third layer |
SiO2 |
Q3 |
2.01 |
Fourth layer |
Ta2O5 |
Q4 |
0.67 |
Fifth layer |
SiO2 |
Q5 |
0.19 |
Sixth layer |
Ta2O5 |
Q6 |
0.85 |
Seventh layer |
SiO2 |
Q7 |
0.43 |
Eighth layer |
Ta2O5 |
Q8 |
0.22 |
Lens substrate Refractive index ns: 1.52 |
[0028] According to Fig. 4 illustrating the spectral reflectance characteristics of the
lens substrate 10 and the antireflective film 20 in Example 1, a maximum reflectance
in a visible light wavelength range of 430 nm to 670 nm was 0.04%. It is found that
the maximum reflectance is suppressed to a very low value in Example 1 as compared
with Comparative Example.
[0029] Conditions of the glass substrate 10 and the antireflective film 20 in Example 2
are illustrated in Table 3. In Example 2, the antireflective film 20 constituted by
eight layers was formed on a surface of the glass lens substrate 10 having a refractive
index n
s = 1.62 at a design main wavelength of λ
0 = 550 nm. The quarter wave optical thicknesses (QWOT) Q
1 to Q
8 of the first layer 21 to the eighth layer 28 with respect to a refractive index n
s = 1.62 of the lens substrate 10 at a design main wavelength of λ
0 = 550 nm satisfy the following formulas (1) to (8). Each layer of the antireflective
film 20 was formed by a vacuum deposition method under heating at 300°C. Particularly,
the second layer 22 to the eighth layer 28 were formed by a vacuum deposition method
using ion assist.
[Table 3]
Example 2 |
Design main wavelength λ0 = 550 [nm] |
Material for layer |
QWOT |
First layer |
MgF2 |
Q1 |
0.95 |
Second layer |
Ta2O5 |
Q2 |
1.89 |
Third layer |
SiO2 |
Q3 |
1.99 |
Fourth layer |
Ta2O5 |
Q4 |
0.66 |
Fifth layer |
SiO2 |
Q5 |
0.13 |
Sixth layer |
Ta2O5 |
Q6 |
1.07 |
Seventh layer |
SiO2 |
Q7 |
0.33 |
Eighth layer |
Ta2O5 |
Q8 |
0.26 |
Lens substrate Refractive index ns: 1.62 |
[0030] According to Fig. 5 illustrating the spectral reflectance characteristics of the
lens substrate 10 and the antireflective film 20 in Example 2, a maximum reflectance
in a visible light wavelength range of 430 nm to 670 nm was 0.04%. It is found that
the maximum reflectance is suppressed to a very low value in Example 2 as compared
with Comparative Example.
[0031] Conditions of the glass substrate 10 and the antireflective film 20 in Example 3
are illustrated in Table 4. In Example 3, the antireflective film 20 constituted by
eight layers was formed on a surface of the glass lens substrate 10 having a refractive
index n
s = 1.72 at a design main wavelength of λ
0 = 550 nm. The quarter wave optical thicknesses (QWOT) Q
1 to Q
8 of the first layer 21 to the eighth layer 28 with respect to a refractive index n
s = 1.72 of the lens substrate 10 at a design main wavelength of λ
0 = 550 nm satisfy the following formulas (1) to (8). Each layer of the antireflective
film 20 was formed by a vacuum deposition method under heating at 300°C. Particularly,
the second layer 22 to the eighth layer 28 were formed by a vacuum deposition method
using ion assist.
[Table 4]
Example 3 |
Design main wavelength λ0 = 550 [nm] |
Material for layer |
QWOT |
First layer |
MgF2 |
Q1 |
0.95 |
Second layer |
Ta2O5 |
Q2 |
1.90 |
Third layer |
SiO2 |
Q3 |
1.99 |
Fourth layer |
Ta2O5 |
Q4 |
0.63 |
Fifth layer |
SiO2 |
Q5 |
0.13 |
Sixth layer |
Ta2O5 |
Q6 |
1.14 |
Seventh layer |
SiO2 |
Q7 |
0.27 |
Eighth layer |
Ta2O5 |
Q8 |
0.29 |
Lens substrate Refractive index ns: 1.72 |
[0032] According to Fig. 6 illustrating the spectral reflectance characteristics of the
lens substrate 10 and the antireflective film 20 in Example 3, a maximum reflectance
in a visible light wavelength range of 430 nm to 670 nm was 0.04%. It is found that
the maximum reflectance is suppressed to a very low value in Example 3 as compared
with Comparative Example.
[0033] Conditions of the glass substrate 10 and the antireflective film 20 in Example 4
are illustrated in Table 5. In Example 4, the antireflective film 20 constituted by
eight layers was formed on a surface of the glass lens substrate 10 having a refractive
index n
s = 1.82 at a design main wavelength of λ
0 = 550 nm. The quarter wave optical thicknesses (QWOT) Q
1 to Q
8 of the first layer 21 to the eighth layer 28 with respect to a refractive index n
s = 1.82 of the lens substrate 10 at a design main wavelength of λ
0 = 550 nm satisfy the following formulas (1) to (8). Each layer of the antireflective
film 20 was formed by a vacuum deposition method under heating at 300°C. Particularly,
the second layer 22 to the eighth layer 28 were formed by a vacuum deposition method
using ion assist.
[Table 5]
Example 4 |
Design main wavelength λ0 = 550 [nm] |
Material for layer |
QWOT |
First layer |
MgF2 |
Q1 |
0.95 |
Second layer |
Ta2O5 |
Q2 |
1.90 |
Third layer |
SiO2 |
Q3 |
1.99 |
Fourth layer |
Ta2O5 |
Q4 |
0.60 |
Fifth layer |
SiO2 |
Q5 |
0.13 |
Sixth layer |
Ta2O5 |
Q6 |
1.23 |
Seventh layer |
SiO2 |
Q7 |
0.22 |
Eighth layer |
Ta2O5 |
Q8 |
0.32 |
Lens substrate Refractive index ns: 1.82 |
[0034] According to Fig. 7 illustrating the spectral reflectance characteristics of the
lens substrate 10 and the antireflective film 20 in Example 4, a maximum reflectance
in a visible light wavelength range of 430 nm to 670 nm was 0.05%. It is found that
the maximum reflectance is suppressed to a very low value in Example 4 as compared
with Comparative Example.
[0035] Conditions of the glass substrate 10 and the antireflective film 20 in Example 5
are illustrated in Table 6. In Example 5, the antireflective film 20 constituted by
eight layers was formed on a surface of the glass lens substrate 10 having a refractive
index n
s = 1.92 at a design main wavelength of λ
0 = 550 nm. The quarter wave optical thicknesses (QWOT) Q
1 to Q
8 of the first layer 21 to the eighth layer 28 with respect to a refractive index n
s = 1.92 of the lens substrate 10 at a design main wavelength of λ
0 = 550 nm satisfy the following formulas (1) to (8). Each layer of the antireflective
film 20 was formed by a vacuum deposition method under heating at 300°C. Particularly,
the second layer 22 to the eighth layer 28 were formed by a vacuum deposition method
using ion assist.
[Table 6]
Example 5 |
Design main wavelength λ0 = 550 [nm] |
Material for layer |
QWOT |
First layer |
MgF2 |
Q1 |
0.95 |
Second layer |
Ta2O5 |
Q2 |
1.90 |
Third layer |
SiO2 |
Q3 |
1.99 |
Fourth layer |
Ta2O5 |
Q4 |
0.57 |
Fifth layer |
SiO2 |
Q5 |
0.13 |
Sixth layer |
Ta2O5 |
Q6 |
1.32 |
Seventh layer |
SiO2 |
Q7 |
0.17 |
Eighth layer |
Ta2O5 |
Q8 |
0.34 |
Lens substrate Refractive index ns: 1.92 |
[0036] According to Fig. 8 illustrating the spectral reflectance characteristics of the
lens substrate 10 and the antireflective film 20 in Example 5, a maximum reflectance
in a visible light wavelength range of 430 nm to 670 nm was 0.06%. It is found that
the maximum reflectance is suppressed to a very low value in Example 5 as compared
with Comparative Example.
[0037] Conditions of the glass substrate 10 and the antireflective film 20 in Example 6
are illustrated in Table 7. In Example 6, the antireflective film 20 constituted by
nine layers was formed on a surface of the glass lens substrate 10 having a refractive
index n
s = 1.62 at a design main wavelength of λ
0 = 550 nm. The quarter wave optical thicknesses (QWOT) Q
1 to Q
8 of the first layer 21 to the eighth layer 28 with respect to a refractive index n
s = 1.62 of the lens substrate 10 at a design main wavelength of λ
0 = 550 nm satisfy the following formulas (1) to (8). Each layer of the antireflective
film 20 was formed by a vacuum deposition method under heating at 300°C. Particularly,
the second layer 22 to the eighth layer 28 were formed by a vacuum deposition method
using ion assist.
[Table 7]
Example 6 |
Design main wavelength λ0 = 550 [nm] |
Material for layer |
QWOT |
First layer |
MgF2 |
Q1 |
0.94 |
Second layer |
Ta2O5 |
Q2 |
1.89 |
Third layer |
SiO2 |
Q3 |
1.98 |
Fourth layer |
Ta2O5 |
Q4 |
0.67 |
Fifth layer |
SiO2 |
Q5 |
0.13 |
Sixth layer |
Ta2O5 |
Q6 |
1.07 |
Seventh layer |
SiO2 |
Q7 |
0.33 |
Eighth layer |
Ta2O5 |
Q8 |
0.26 |
Ninth layer |
Al2O3 |
Q9 |
0.14 |
Lens substrate Refractive index ns: 1.62 |
[0038] According to Fig. 9 illustrating the spectral reflectance characteristics of the
lens substrate 10 and the antireflective film 20 in Example 6, a maximum reflectance
in a visible light wavelength range of 430 nm to 670 nm was 0.04%. It is found that
the maximum reflectance is suppressed to a very low value in Example 6 as compared
with Comparative Example.
[0039] In this way, according to the configuration of the embodiment, it is possible to
form the antireflective film 20 with a low reflectance and a small loss of light corresponding
to the lens substrates 10 having various refractive indices. That is, it is possible
to form the antireflective film 20 using a high refractive index material which has
been relatively difficult to use conventionally, and it is possible to expand the
degree of freedom of the configuration of the projection lens LN. As a result, it
is possible to effectively suppress a decrease in the transmittance of the whole system
of the projection lens LN, and it is possible to flexibly cope with an increase in
the number of constituent lenses.
[0040] Furthermore, in the antireflective film 20, each of the second layer 22, the fourth
layer 24, the sixth layer 26, and the eighth layer 28 is formed of any one of Ta
2O
5, LaTiO
3, a mixture of Ti
2O
3 and ZrO
2, and a mixture of ZrTiO
4 and ZrO
2. Therefore, it is possible to form the antireflective film 20 with a small loss of
light by a vacuum deposition method under a relatively high temperature environment
of, for example, 300°C. There is a risk that practical strength may be lowered in
a case where MgF
2 used in the first layer 21 is formed in a low temperature environment. Therefore,
according to the configuration of the present embodiment, it is possible to increase
the strength of the first layer 21.
[0041] The antireflective film 20 desirably has a maximum reflectance of 0.2% or less in
a wavelength range of 430 nm to 670 nm. This makes it possible to obtain the antireflective
film 20 sufficient for application to a recent increase in the number of pixels of
an image in the projection lens LN.
[0042] In addition, three or more types of glass materials among glass materials classified
into five types satisfying the above formulas (9) to (13) regarding a refractive index
n
s are used as the lens substrate 10. Therefore, even with the projection lens LN obtained
by combining the lens substrates 10 formed of various glass materials for thirty lenses,
it is possible to form the antireflective film 20 with a low reflectance and a small
loss of light. This makes it possible to further widen the degree of freedom of the
configuration of the projection lens LN.
[0043] The embodiment of the present invention has been described above. However, the scope
of the present invention is not limited to the embodiment, and the present invention
can be carried out by making various modifications to the embodiment without departing
from the gist of the invention.
Industrial Applicability
[0044] The present invention can be used in a projection lens.
Reference Signs List
[0045]
- 1
- Glass
- 10
- Glass substrate
- 20
- Antireflective film
- 21
- First layer
- 22
- Second layer
- 23
- Third layer
- 24
- Fourth layer
- 25
- Fifth layer
- 26
- Sixth layer
- 27
- Seventh layer
- 28
- Eighth layer
- LN
- Projection lens